This application is a continuation of International Application No. PCT/JP02/02592, filed Mar. 19, 2002, which claims the benefit of Japanese Patent Application No. 080504/2001,filed Mar. 21, 2001.
1. Field of the Invention
The present invention relates to a drive circuit for a light emitting device for use in an image display apparatus, more particularly to a drive circuit for a light emitting device of active matrix type for driving a light-emitting device such as an organic or inorganic electroluminescent (hereinafter called xe2x80x9cELxe2x80x9d)device or a light-emitting diode (hereinafter called xe2x80x9cLEDxe2x80x9d), and a display panel of active matrix type utilizing such drive circuit.
2. Related Background Art
A display utilizing light-emitting devices such as organic or inorganic EL devices or LEDs arranged in an array and displaying a character by a dot matrix method is widely utilized in television, portable information terminal etc.
Such display based on the light-emitting device is currently interested because of the features, in comparison with the display utilizing liquid crystal, of the absence of a light source for illumination from the rear and a wider viewing angle. In particular, the display of so-called active matrix type, in which a static drive is executed by the combination of transistors and the aforementioned light-emitting devices, is currently attracting attention because of advantages of a higher luminance, a higher contrast and a higher definition, in comparison with the display of simple matrix drive based on time-shared drive.
Also for providing an image with gradation in the organic EL device, there can be conceived an analog gradation method, an area gradation method and a time gradation method as already known in the prior art:
As an example of the conventional configuration, a simplest display device utilizing two thin film transistors (hereinafter called xe2x80x9cTFTxe2x80x9d)per pixel is shown in FIGS. 6 and 7. In FIG. 6, there are shown an organic EL element 101, TFTs 102, 103, a scanning line 107, a signal line 108, a power supply line 109, a ground potential 110, and a memory capacity 111 utilizing a capacitor.
The circuit shown in FIG. 6 functions in the following manner. When TFT 102 is turned on by the scanning line 107, an image data voltage from the signal line 108 is accumulated in the memory capacity 111. When the scanning line 107 is turned off to turn off TFT 102, the above-mentioned voltage continues to be applied to the gate of TFT 103 whereby TFT 103 remains in the turned-on state.
On the other hand, the source electrode of TFT 103 is connected to the power supply line 109, while the drain electrode is connected to one of the electrodes of the light-emitting element, and the gate electrode receives the image data voltage at the drain electrode of TFT 102, whereby the current between the source electrode and the drain electrode is controlled by the above-mentioned image data voltage. The organic EL element, being connected between the power supply line 109 and the ground potential, emits light corresponding to the aforementioned current.
Since the amount of current depends on the gate potential of TFT 103, the light emission intensity is regulated by changing the current characteristics in analog manner, utilizing an area (saturation area) where the source current as a function of the gate potential (Vg-Is characteristics) shows an upshift.
As a result, the light emission intensity of the organic EL element can be controlled and the display involving gradation can be realized. Such gradation representing method, utilizing an analog image data voltage, is called analog gradation method. In such method, the image data signal has to be adjusted in the gamma (xcex3) characteristics according to the voltage-luminance characteristics of the organic EL element.
It is advantageous also for the light-emitting device, as in the liquid crystal display device or in the CRT, to enable gradational display by varying the light emission intensity of each pixel in order to achieve moving image display for the monitor of the personal computer or the television and also in order to ensure compatibility with the CRT. Also there will be obtained an advantage in cost, because of simplification in the driving system.
The aforementioned TFT currently includes that of amorphous silicon (a-Si) type and that of polycrystalline silicon (p-Si) type, but the latter is becoming more popularly employed because it shows a higher charge mobility enabling a finer configuration of the element and also because the progress in the laser working technology enables to execute the manufacturing process at a lower temperature. However, the polycrystalline silicon TFT is often influenced by the crystal grain boundary constituting the element, and tends to show a significant fluctuation from element to element in the Vg-Is current characteristics in the aforementioned saturation area. Therefore, such display device is-associated with a drawback of showing unevenness in the display, even if the video signal voltage entered into the pixels is uniform.
Also, the present TFT is mostly used as a switching element in an area where the drain voltage becomes constant as a function of the source voltage (such area being called a linear area) under the application of a gate potential considerably higher than the threshold voltage of the transistor, so that the aforementioned fluctuation in the saturation area is not much experienced.
On the other hand, an area gradation method is proposed in the reference AM-LCD2000, AM3-1. In this method, each pixel is divided into plural sub-pixels, each of which is on-off controlled to represent the gradation by the area of turned-on sub-pixels in the pixel.
In such method, the TFT can be utilized in the aforementioned linear area where the drain voltage becomes constant as a function of the source voltage, under the application of a gate potential much higher than the threshold voltage, so that the TFT can be used in a stable range of the characteristics and the light emission intensity of the light-emitting element is also stabilized. In such area gradation method, each element is on-off controlled and emits light at a constant intensity without gradational change, and the gradation is controlled by the area of the light-emitting sub pixels.
In this method, however, there can only be obtained digital gradation levels depending on the method of division of the sub pixels, and, in order to increase the number of gradation levels, it is required to increase the number of sub pixels with a reduction in the area thereof. However, even if the transistors are made smaller with the use of polycrystalline silicon TFTs, the area of the transistor portion in each pixel erodes the light-emitting area, thereby lowering the aperture rate of each pixel and reducing the light emission intensity of the display panel. Thus the luminance the gradational performance are in a trade-off relationship in which an increase in the aperture rate results in a decrease in the gradational performance, whereby it is difficult to improve the gradational performance.
In a time gradation method controls the gradation by the light-emitting time of an organic EL element, as reported in 2000SID36.4L.
FIG. 7 is a circuit diagram showing an example of a pixel portion of a conventional display panel employing the time gradation method. In FIG. 7 there are shown an organic EL element 101, TFTs 102 to 104, a scanning line 107, a signal line 108, a power supply line 109, a ground potential 110, a memory capacity 111 and a reset line 112.
In the time gradation method utilizing such circuit configuration, when TFT 103 is turned on, the organic EL element 101 emits light at the maximum intensity by the voltage from the signal line, while TFT 103 repeats on and off within a field time by TFT 104 and the gradation is represented by such light-emitting time.
Also in this method, the light-emitting time is regulated by selecting one of plural light-emitting periods. For example, in case of gradational display of 8 bits (256 levels), the light emitting time is selected from 8 sub-field periods having a ratio of 1:2:4:8:16:32:64:128. Immediately before each sub-field period, there is provided an addressing period for the scanning lines of all the pixels, for selecting the emitting or non-emitting state in such sub-field period. After such addressing period, the voltage of the power supply lines 109 is changed simultaneously to cause light emission over the entire display panel.
Consequently, since the addressing period is basically not used for display, the effective light-emitting period within a field, in case of N-bit gradational display, is given by:
effective light-emitting period=(a field period) xe2x88x92(addressing periodxc3x97N in an image).
Therefore the light-emitting time becomes short in relative manner and results in a decrease in the light emission intensity of the display panel for the observer.
For this reason it becomes necessary to compensate the light emission amount in the entire field by increasing the light emission amount in each sub field, but such increase necessitates an increase in the light emission intensity of each light-emitting element, leading eventually to a reduction in the service life thereof. Also in comparison with the ordinary liquid crystal display (LCD) which requires only one addressing operation per field, there are required addressing operations corresponding to the number of bits of gradational levels, so that an addressing circuit of a higher speed is required and an increase in the electric power consumption is unavoidable.
The object of the present invention is to improve the conventional technologies explained in the foregoing, to provide a novel circuit configuration of the pixel transistors for a novel active matrix light-emitting device, and to provide a display panel superior to that of the conventional art.
A principal feature of the present invention resides in a circuit configuration of the light-emitting element of active matrix type in which a switching element is provided electrically parallel to the light-emitting element.
A second feature of the present invention resides in a circuit configuration of the light-emitting element of active matrix type in which a second switching element is provided at a side, closer to a constant current source, of the aforementioned light-emitting element.
The above-mentioned object can be attained, according to the present invention, by a drive circuit for a light-emitting device of active matrix type having a scanning line and a signal line in a matrix arrangement on a substrate and also having at least a light-emitting element in the vicinity of the crossing point of the scanning line and the signal line, the drive circuit comprising a constant current source connected to a driving electric power source, a light-emitting element provided serially to the constant current source, and a first switching element provided serially to the constant current source and electrically parallel to the light-emitting element.
In a preferred embodiment of the drive circuit of the present invention, the aforementioned first switching element is a first thin film transistor comprised of three electrodes of a source electrode, a drain electrode and a gate electrode.
Also, the drive circuit of the present invention includes as a preferred embodiment thereof a memory circuit capable of accumulating the image data signal. More specifically, the drive circuit of the present invention as a preferred embodiment comprises a memory circuit comprised of a second thin film transistor which has a gate electrode connected to the scanning line, a source electrode connected to the signal line, and a drain electrode, and a first memory capacitance.
Also, a preferred embodiment of the drive circuit of the present invention executes on-off control utilizing the aforementioned configuration of the drive circuit. More specifically, the drive circuit of the present invention in a preferred embodiment thereof executes on-off control of the light-emitting element by controlling the current in the aforementioned first switching element and the current amount in the aforementioned light-emitting element according to the information from the scanning line and the signal line.
The present invention further includes a preferred embodiment in which the aforementioned configuration of the drive circuit is utilized for gradational display. For this purpose, there may be employed the time gradation method or the analog gradation method. More specifically, the drive circuit of the present invention, in a preferred embodiment thereof, executes gradational display by controlling the light-emitting time, and, in another preferred embodiment, controls the light emission intensity of the aforementioned light-emitting element by controlling the current amount in the aforementioned first switching element and the current amount in the light-emitting element, according to the information from the scanning line and the signal line.
The preferred embodiment of the present invention further includes an improvement on the aforementioned configuration of the drive circuit. More specifically, the drive circuit of the present invention is preferably provided with a second switching element between the second electrode of the aforementioned light-emitting element and the aforementioned constant current source, and is more preferably adapted to execute on-off control of the light-emitting element by the switching operation of the second switching element. There is further preferred a configuration in which the second switching element is a third thin film transistor comprised of three electrodes, namely a source electrode, a drain electrode and a gate electrode. Also the drive circuit of the present invention, provided with the aforementioned second switching element, is preferably provided with a second memory circuit comprised of a fourth thin film transistor and a second memory capacitance, in which the output of such memory circuit is connected to the gate electrode of the aforementioned third thin film transistor.